Manufacture of cellulose acetate



Patented Apr. 7, 19 316 NETE STTES NT OFFIfiE Ralph H. McKee, JerseyCity, and Earle H. Morse, Nutley, N. J

No Drawing. Application October 6, 1930, Serial No. 486,858

15 Claims.

This invention relates to an improved process of manufacturing celluloseacetate which is particularly adapted for operation as a continuousprocess although, if desired, it may be operated as a batch process.

In its broad aspects the process consists in softening the fibers ofcellulosic material by pretreatment such as with acetic acid or amixture of acetic acid and other agents, such as an oxidizing agent,followed by the treatment of the cellulosic material in a bathcontaining acetic anhydride, an inert diluent which is a non-solvent ofcellulose acetate and cellulose and which is a diluent for acetic acidand acetic anhydride, such as toluene, and an acetylating catalyst.There results from this treatment, depending upon the time, temperature,concentration of reagents and like factors, a cellulose acetate of adefinite solubility. Ordinarily, the treatment is allowed to continueuntil the resulting cellulose acetate is soluble in chloroform. However,by modified treatments cellulose acetate of other types andcharacteristics may be prepared. For example, cellulose acetate ofdifferent solubilities, such as products soluble in acetic acid,acetone, mixtures of alcohol and tetrachlorethane or chloroform, etc.may be prepared.

In the cellulose acetate industry it is customary to employ a cellulosewhich has been treated with caustic soda, washed, and dried to nearlybone dryness. In the preparation of cellulose acetate from suchmaterial, it is customary to subject the material to a pretreatment withacetic acid or acetic acid vapor. In typical commercial practicefollowing this treatment a given amount of the material, say 100 parts,is acetylated by use of 850 to 400 parts each of glacial acetic acid andacetic anhydride, and 2 to 5 parts of sulfuric acid or other catalyst.In this process, the cellulose is commonly used either in the form ofshredded cotton cellulose or shredded wood pulp cellulose. In this andfollowing paragraphs when parts are designated parts by weight aremeant.

In the present process, we prefer to employ the cellulose in the form ofa strip, such as a strip of cotton cloth or. a strip from a roll ofcotton or wood pulp cellulose. By employing the cellulose in such formit is possible to conduct the process as a continuous process mostadvantageously, as will hereinafter be more readily apparent. However,it will be obvious that the process may be employed as a batch processand that the cellulosic material in the shredded form commonly employedmay be used if desired. However, in view of our preference for employinga strip of cellulose material and for operating the process as acontinuous one, such operation of the process will be hereinafterdescribed as the preferred embodiment thereof.

In the practice of the process, a strip of cellulosic material is passedinto a bath of acetic acid or a bath containing acetic acid and anoxidizing agent. Instead of treating the cellulosic material with aliquid bath, the reagents may be used in vapor phase if desired. We havefound that a somewhat dilute acetic acid, such as to per cent. aceticacid, may be employed in this pretreatment step, but our preference isto employ approximately per cent. acetic acid since the time ofpretreatment is materially shortened thereby. As stated, we have foundit advantageous to add an oxidizing agent to the pretreating bath and assuch agent we prefer to employ nitric acid. Other materials such ashalogens, chromic acid, manganese compounds and the like may also beemployed. When an oxidizing agent of the character referred to isemployed, this material is preferably used in small amounts, say two percent. on the basis of the cellulosic material to be treated. While it isnot definitely known why the addition of oxidizing agents to thepretreating bath is advantageous it appears that the speed of softeningof the cellulose fiber is increased by the presence of the oxidizingagent.

The length of the tank containing the pretreating bath and the speed ofpassage of the cellulose material therethrough are so chosen that thecellulose will be in contact with the acetic acid long enough for theacetic acid to penetrate into substantially every fiber. The length oftime depends upon the form of cellulose strip (tightness of twist of thecotton thread in the cloth, etc), upon the temperature of the operation,upon the amount of secondary reagents present in the bath after theprocess has been in operation for some period, and the strength of theacetic acid. In actual practice, we have found that a period ofapproximately two hours is required for the pretreatment operation whena bath containing 250 parts of acetic acid of approximately 100 percent. strength, per 100 parts of cellulose to be treated is employed,and the process is operated at a temperature of approximately 25 C. Thistemperature is the preferred temperature of operation but may be widelyvaried Without serious disadvantage. For example,

the process may be successfully operated at a temperature of from to 100C.

Instead of employing a pretreatment bath. of the character set forthabove, we may use, if desired, acetic acid diluted with an inert solventsuch as the discarded contents of the acetylating bath hereinafterdescribed in detail or instead the distilled portion of same containingacetic acid. The ingredients other than acetic acid present in suchdiscarded acetylating bath do not prevent the penetration of the aceticacid into the cellulose fi er and in some instances it may be foundeconomical to use this discarded acctylating bath for the pretreatment.

After the desired period of treatment of the cellulosic material in thepretreatment bath the cellulose strip is passed between press rolls andthe excess of pretreating liquid is pressed out and returned to thepretreatment bath. The pretreated cellulose strip then subjected to anacetylation treatment. This involves passing the. strip of cellulosematerial into a bath containing an inert diluent which is a non-solventof cellulose acetate or cellulose, such as toluene, acetic anhydride,acetic acid (carried over from pretreatment bath and formed by theacetylation), and an acetylating catalyst, such as nitrosyl-sulfuricacid. Other catalysts such as sulfuryl chloride, a mixture ofconcentrated nitric and sulfuric acids, a mixture of nitrous andsulfuric acids, and the like may be employed in place of thenitrcsyl-sulfuric acid. Only small amounts of such catalysts are needed.Other non-solvents of cellulose acetate and cellulose such as benzene,carbon tetrachloride and the like or their mixtures may be employed asthe! inert diluent provided they are miscible with but without action onacetic acid and acetic anhydride, but we prefer to employ toluene. Wedesignate such liquids by the term inert diluents.

The proportions of ingredients in the acetylating bath may be variedconsiderably. We prefer, however, to employ a bath in which at least 50per cent. of the volume thereof consists of an inert diluent, preferablytoluene. In actual practice, we have found it advantageous to employ,for each 100 parts of cellulose treated, a bath containing from 260 to.500 parts of acetic anhydride and ie-*3 to 1,600 parts of toluene. Ofthis bath only a part will be used up or removed by the acetyiatedcellulose. Acetic acid will be present in an amount depending upon theamount formed by reaction and the amount carried over from thepretreating bath. The amount of catalyst employed is small, for exampleabout per cent. on the basis of the cellulosc to be treated. Th time oftreatment of the material in the acetylating bath at a temperature of C.is ordinarily about 14 hours. The product obtained under such conditionsis completely soluble in chloroform.

By employing somewhat more of the catalyst, say about one per cent, andoperating the proccss at a somewhat higher temperature, say 50 C. theoperation can be carried out in from 1 to 2 hours. In this case,acetylation takes place so rapidly that in the first stage of theoperation cooling may be desirable.

While the acetylation operation may be conducted in a single bath asdescribed above, we prefer to employ a plurality of acetylation bathsand pass the strip suc essively through the baths. When such procedureis followed, we prefer to use acetylating baths of varying proportions,the

concentration of the acetic anhydride in the several baths progressivelyincreasing and the concentration of acetic acid preferably decreasing.We have found that by such a treatment a product of superior qualitiesis produced.

In practising the process wherein two acetylation baths are employed, wehave found it advantageous to use in the first bath an acetylatingmixture consisting of approximately 400 parts of toluene, or other inertdiluent, 200 parts of acetic anhydride, and approximately part ofnitrosyl-sulfuric acid. The strip of cellulose (100 parts) is treated insuch a bath at a temperature of approximately 40 C. for approximatelytwo hours after which the strip is passed through press rolls to removethe excess of treating liquid. The strip is then passed through a secondacetylating bath containing approximately 350 parts of acetic anhydride,800

parts of an inert diluent such as toluene, and

approximately part of nitrosyl-sulfuric acid. The strip is treated inthe second acetylating bath at a temperature of approximately 40 C. forapproximately four hours, after which the strip is passed through pressrolls to remove as much of the acetylating bath as practicable. Theresulting material is chloroform soluble. As is well known, in theprocess of acetylation one molecule of acetic acid is formed and left inthe acetylating bath from each molecule of acetic anhydride used in theacetylation.

If it is desired to render such a cellulose acetate soluble in acetone,the strip is passed into a bath in which partial hydrolysis will occurwhereby the material is rendered acetone soluble.

This bath may consist of hot dilute acetic acid or of other materialscommonly used for this purpose. This step of the process is conventionaland any suitable known method may be employed. Following the hydrolysistreatment, the cellulose acetate strip is preferably passed throughtanks carrying clear water (or water containing a small amount ofalkali) wherein the material is thoroughly washed. It is then driedpreferably by passing the strip over heated drums. The resultingmaterial may be rolled up for storage or passed directly to tanks whereit is dissolved in acetone and clarified by filtration, following whichthe solution thus prepared may be delivered to spinning cells to producecellulose able to recover the mixture of the inert diluent and aceticacid in pure form. For this reason, we prefer to employ toluene as theinert diluent because this material can be very readily recovered inpure form, as can the acetic acid when toluene is used. This recoverycan be inexpensively accomplished by distillation without appreciableloss by making use of the fact that toluene and acetic acid form aconstant boiling mixture, the composition of which varies with thepressure at which the mixture is distilled. In other words, by simplydistilling off part of the material at one pressure, say atmosphericpressure, at which the constant boiling mixture contains about per cent.acetic acid, and then reworking part by distilling at another pressure,say at a pressure of three atmospheres, at which the constant boilingmixture contains about 50 per cent. of acetic acid, possible to separatethese materials so that the acetic acid can be either sold as glacialacetic acid or recovered for reuse in the process. Actually, there issome accumulation of acetic acid in the process, so that it will benecessary to dispose of a part of it. The points at which thedistillation cuts are made will depend upon the composition of theexhausted acetylation bath. However, this will be apparent to thoseskilled in the art.

If desired the constant boiling mixture carrying toluene and acetic acidrecovered by the distillation process referred to above can be employedas the pretreating material with or without an oxidizing agent, insteadof acetic acid alone. It has been demonstrated in actual tests thatunder some conditions of operation it may be found advantageous tofollow such procedure.

A modification of the process can be carried out without the use of aninert diluent by using the pretreatment step of softening with aceticacid, partially acetylating the moving cellulosic material in a mixtureof acetic acid and acetic anhydride and catalyst to give a lowercellulose acetate, say one having an acetyl content of 20 per cent. to30 per cent. calculated as acetic acid, then passing the moving strip ofpartially acetylated cellulose into a second bath containing a catalystand rich in acetic anhydride and before acetylation is complete passingit through press rolls so set that sufficient acetic anhydride andcatalyst will be left in the cellulosic strip to complete theacetylation but not sufficient acetic acid and excess acetic anhydrideto bring the cellulose acetate into solution. The strip of cellulosicmaterial can be loosely rolled up and allowed to mature until theacetylation is complete. After partial hydrolysis to render it acetonesoluble it will then be washed with water to remove the acid. It will benoted that the same principle is used in this modification as when theinert diluent is used, i. e., the acetylation of a moving cellulosicmaterial by acetic anhydride in the presence of an acetylating agentunder conditions such that the cellulose acetate produced does not gointo solution in the mixture of acetic acid and acetic anhydride usedfor acetylation but remains in its original form.

The term inert diluent as employed in the specification and claims isintended to include materials which when present in the acetylating bathprevent the solution of the cellulose acetate or cellulose by the aceticacid or acetic anhydride and which are inert with respect to acetic acidand acetic anhydride though miscible therewith, i. e. soluble in them.

The term acetylating catalyst as employed in the specification andclaims is intended to cover nitrosyl-sulfuric acid, sulfuryl chloride, amixture of concentrated nitric and sulfuric acids, a mixture of nitrousand sulfuric acids, and similarly acting catalysts as described above.While the materials referred to are termed catalysts there is someevidence that the materials themselves are not the actual activatingcatalysts but instead act with the acetylating mixture to form a newcompound which is the true catalyst. However, in a broad sense they maybe considered as catalysts and are so considered herein.

As stated above, we prefer to operate the process by passing a strip ofcellulosic material through the various baths, rolls, driers, etc.described above and this may be accomplished by regulating conditions sothat the strip may move continuously throughout the process. However,the movement may be intermittent. For example, the strip may be movedforward for a desired distance and then stopped for a desired period oftime. It will be understood that the term moving is intended to includeboth of these types of movement.

While we have described in detail the preferred embodiment of ourinvention it is to be understood that the details of procedure may bevariously modified without departing from the spirit of the invention orthe scope of the subjoined claims.

We claim:

1. The process of manufacturing cellulose acetate which comprisespassing a moving strip of cellulose, prior to acetylation thereof,through a treating bath comprising acetic acid and an oxidizing agent,passing said strip out of said bath, and thereafter successively passingthe moving strip into, through, and out of a plurality of separatebaths, each containing an acetylating mixture, to convert the celluloseto cellulose acetate.

2. The process of manufacturing cellulose acetate which comprisespassing a moving strip of cellulose, prior to acetylation thereof,through a treating bath comprising acetic acid and an oxidizing agentcomprising nitric acid, passing said strip out of said bath, andthereafter successively passing the moving strip into, through, and outof a plurality of separate baths, each containing an acetylatingmixture, to convert the cellulose to cellulose acetate.

3. The process of manufacturing cellulose acetate which comprisespassing a moving strip of cellulose, prior to acetylation thereof,through a treating bath comprising acetic acid, an oxidizing agent, andan inert diluent, passing said strip out of said bath, and thereaftersuccessively passing the moving strip into, through, and out of aplurality of separate baths, each containing an acetylating mixture, toconvert the cellulose to cellulose acetate.

4. The process of manufacturing cellulose acetate which comprisessubjecting a moving strip of cellulose to the action of a treating bathcomprising acetic acid and an oxidizing agent, moving said strip out ofsaid bath, pressing the excess of said treating mixture out of saidstrip, and passing said strip into, through, and out of a separate bathcomprising an acetylating mixture containing acetic anhydride and aninert diluent.

5. The process of manufacturing cellulose acetate which comprisestreating a moving body of cellulose with acetic acid of greater than 80per cent. strength, and thereafter moving the body of material through aseries of independent acetylating baths, each comprising an acetylatingmixture containing acetic anhydride, an inert diluent, and anacetylating catalyst.

6. The process of manufacturing cellulose acetate which comprisessubjecting cellulose to the action of a bath comprising acetic acid ofgreater than 80 per cent. strength, removing the material from saidbath, thereafter treating the material in a separate bath comprising anacetylating mixture containing acetic anhydride, an acetylating catalystand an inert diluent, removing said material from said second namedbath, thereafter treating the partially acetylated material in a thirdbath comprising an acetylating mixture containing acetic anhydride, anacetylating catalyst, and an inert diluent, removing said material fromsaid third named bath, pressing out the excess acetylating mixture, andpermitting the resulting material to stand for completion ofacetylation.

7. The process of manufacturing cellulose acetate which comprisestreating a moving body of cellulose with acetic acid of greater than 80per cent. strength, and thereafter moving the body of material through aseries of independent acetylating baths, each comprising an acetylatingmixture containing acetic anhydride, toluene and an acetylatingcatalyst.

8. The process of manufacturing cellulose acetate which comprisestreating a moving body of cellulose with acetic acid of greater than 80per cent. strength, and thereafter moving the body of material through aseries of independent acetylating baths, each comprising an acetylatingmixture containing acetic acid, acetic anhydride, toluene, andnitrosyl-sulfuric acid.

9. In a process of manufacturing cellulose acetate the improvement inacetylating treatment which comprises successively introducing thematerial to be treated into and removing the material from a pluralityof separate acetylating baths, each containing acetic acid, aceticanhydride and an inert diluent, in which series of baths theconcentration of the acetic anhydride progressively increases and theconcentration of the acetic acid progressively decreases.

10. The herein described process of manufacturing cellulose acetatewhich comprises passing a moving strip of cellulose through a bathcontaining acetic acid of greater than 80 per cent. strength and anoxidizing agent comprising nitric acid, pressing out the excess oftreating mixture from the strip, passing the treated strip through asuccession of acetylation baths each containing acetic acid, aceticanhydride, toluene, and nitrosyl-sulphuric acid, the toluene in each ofsaid baths constituting more than 56 per cent. of the bath, and theconcentration of the acetic acid in said baths progressively decreasingand the concentration of acetic anhydride progressively increasing,pressing out the excess of acetylating mixture from the strip, andpassing the thus treated strip through a hydrolyzing bath for partialhydrolysis of the material to render it soluble in acetone.

11. In a process of manufacturing cellulose acetate the improvement inacetylating cellulose without putting it into solution which comprisesthe steps of treating such material with acetic acid, removing theexcess acetic acid from the material, and thereafter successivelyintroducing the material to be treated into and removing the materialfrom a plurality of separate acetylating baths, each containing aceticanhydride, an inert diluent, and an acetylating catalyst, the excessacetylating mixture being removed from the material after treatmentthereof in one of said series of acetylating baths and prior totreatment thereof in the next of said series of said acetylating baths.

12. The process of manufacturing cellulose acetate which comprisesacetylating cellulose to render the material chloroform-soluble bysuclose without putting it into solution which comprises successivelyintroducing the material to be treated into and removing the materialfrom a plurality of separate acetylating baths, each containing aceticanhydride, an inert diluent,

and an acetylating catalyst, to produce a substantially completelyacetylated cellulose.

14.111 a process of manufacturing cellulose acetate, the improvement inacetylating cellulose Without loss of its fibrous character whichcomprises successively introducing the material to be treated into andremoving the material from a plurality of separate acetylating baths,each containing acetic anhydride, an inert diluent and an acetylatingcatalyst, and permitting the resulting material to stand for completionof acetylation, and treating the acetylated material in a hydrolyzingbath for partial hydrolysis of the material to render it soluble inacetone.

15. In a process of manufacturing cellulose acetate, the improvement inacetylating cellulose without putting it into solution which comprisessuccessively introducing the material to be treated into and removingthe material from a plurality of separate acetylating baths, eachcontaining acetic anhydride, an inert diluent, and a catalyst, toproduce a substantially completely acetylated cellulose, said catalystbeing selected from the group consisting of nitrosylsulfuric acid,sulfuryl chloride, a mixture of concentrated nitric and sulfuric acids,and a mixture of nitrous and sulfuric acids.

RALPH H. MCKEE. EARLE H. MORSE.

